Some plant tips are personal. Should you prune in fall or spring? Choose a purple flower or a yellow one? Yet when it comes to soil volume for trees, there is no debate. It’s a topic that has been studied for decades and directly determines whether your urban tree survives more than a few years. Street trees are one of the most important elements in our cities, yet they are constantly stressed and short-lived due to poor planting conditions The average lifespan of a street tree is only 7 to 10 years. For trees in containers, that drops to just 2 to 5 years. A Seattle study showed that 80 percent of unirrigated newly planted street trees died within two years.
There is another widespread agreement in urban forestry: if your tree cannot achieve a full canopy, you cannot guarantee the benefits we all promise to our clients: carbon storage, cooling shade, stormwater capture. Poor crown size means the ecosystem services we sell remain purely theoretical. If we want to back up those promises, we must do better underground.
Soil Equals Water Storage
A tree’s need for soil volume is really a need for water storage. Soil is a physical medium for roots to grow, but its main job is to hold enough water to meet the tree’s needs. Water moves from the soil into the roots, up the trunk, and out through the leaves where almost all of it evaporates directly into the air, allowing the tree to complete its cycle of photosynthesis.
When soil moisture is low and the air is dry, the tree responds. Stomata close, halting photosynthesis. Leaves may roll or wilt, and leaf drop can occur, all to slow water loss. But with inadequate soil and insufficient water, these stress signals never stop. Add in the urban heat island effect, which increases water loss even more, and you make these terrible growing conditions even worse. These stressed trees will be weak, poorly formed, and very susceptible to pests and diseases.
Because we often have no rain from April through November in Southern California, soil and water limitations are even more critical to address. Picture this: a small street tree needs about 35 gallons of water each day just to survive. A single cubic foot of healthy, loamy soil holds a little more than one gallon of water. So if you plant that tree in a 48-cubic-foot pit (four feet by four feet by three feet deep) you can keep it alive, but you will have to irrigate almost daily for it to remain healthy, and it probably will never grow much larger than when you planted it. By contrast, a large, healthy tree can use up to 250 gallons every day, which simply is not possible if the soil volume cannot hold enough water to meet that demand (which would require daily watering in a pit ten feet by ten feet by three feet deep).
Roots Spread Wide, Not Deep
Most tree roots stay in the top three feet of soil. Going deeper than that does not help. But in Los Angeles, you really need the full three feet, especially on-structure. Deeper soil plus mulch keeps temperatures low and cuts evaporation, which is critical in our hottest season. A coarse mulch layer three to four inches deep can conserve more than 80 percent of rain or irrigation water.
And you must be careful planting under the canopy. Turf or thirsty groundcovers like Trachelospermum jasminoides can steal soil moisture from tree roots, since they share the same soil. I have witnessed this firsthand on many projects. At my parents’ house, their Coast Redwood was badly browning at the tips until we removed the turf and widened the mulch ring at its base. The tree recovered almost instantly. On another project site, several Magnolia grandiflora were declining in raised planters. I had read a few articles about how they didn’t make good container plants and we were just about ready to remove and replace them, but an arborist pointed out the Trachelospermum surrounding the trees, stating that they might be stealing moisture from the root zone. Once the jasmine was removed, the trees made a full recovery. We may want to surround our trees with beautiful plants, but we must remember that supplemental irrigation is required to do so. These examples show how easily tree health can suffer when the root zone is restricted or outcompeted, and how quickly a simple fix like widening mulch or removing an aggressive groundcover can restore balance.
Provide and Connect the Soil
So how much soil do you need? A general estimate is about two to three cubic feet of soil for every square foot of tree canopy you want to achieve. This requires doing some simple math, but it’s worth it in the long run if you get healthy trees out of it. I can guarantee the first time you do the calculation; you might be a little scared of how much soil is required. You’ll want to do your best to achieve that number, however possible, if you want your trees to grow beyond installation size.
A mulched tree pit is better than nothing but connecting soil zones works far better. Linking multiple trees under the sidewalk or along parkway strips gives roots more room to grow. There are many methods to achieve this – like using Structural Soil, Silva Cells, or the Stockholm Tree Pit System (
https://stockholmtreepits.co.uk/
) to expand root space under paving without damaging the surface. Silva Cells have been proven to work better than basic Structural Soil but typically cost more up front, and the Stockholm Tree Pit System is new and claims to work better than the other two methods. You may want to work with an arborist or soil specialist if you are unsure how to best guarantee the health of your trees. In the long run, any method you can use to allow roots to spread further and comingle with one another, the healthier and happier your trees are going to be.
The Bottom Line
Trees in tiny soil prisons fail first when water runs out. Bigger soil volumes lead to bigger canopies, stronger roots, and real shade. In the context of Los Angeles’s climate, investing in proper soil volumes is especially critical. It underpins the city’s ability to grow a robust urban forest that can endure heat and drought. A helpful rule of thumb for healthy, long-lived trees is to provide at least 300 cubic feet for small trees (mature height under 25 feet), 600 cubic feet for medium trees (mature height between 25 and 40 feet), and 1,000 cubic feet for large trees (mature height over 40 feet). Whenever possible, exceeding these minimums will improve tree vigor and longevity. If you can’t reach these numbers, choose a smaller tree, link up your tree pits, and always opt for irrigation. The trees will thank you.
Key References
International Society of Arboriculture (ISA) White Paper: Specifying Soil Volumes to Meet the Water Needs of Mature Urban Street Trees and Trees in Containers by Patricia Lindsey and Nina Bassuk
Patrick Francis MacRae’s follow-up study Evaluation of Soil Volume Requirements for Urban Trees with Nina Bassuk
Also check out James Urban’s Soil Volume Recommendations for Vines, Hedges, and Palms